The short answer is possibly. For many years the Mac has survived the constant barrage of PC viruses and worms. However, as the platform grows in popularity, spreading outside of creative circles into most people's lives and work, it becomes a target for virus writers.
OS X is still a much more secure and solid operation system than its competitors, which needs user verification before it installs anything. Common sense will prevail; don't open or install anything that you weren't waiting for or that looks either too good to be true, or dodgy!
In all my years of computing, I have only heard of a small handful of malicious codes in the wild (virus, worms etc), but my strategy, as well as Apple's, is better safe than sorry. Apple themselves recommend Intego, McAfee and Symantec. Read this article for more details.
Addendum 1: Apple have since removed the article saying that it was old and inaccurate.
Addendum 2: Here's an interesting article on this subject at TidBITS.